1. Field of the Invention
The present invention relates to a bonding wire for use in a semiconductor device, particularly in connection between a semiconductor element and an external lead.
2. Prior Art
As a method for electrically connecting an electrode of a semiconductor element and an external lead, there has heretofore been extensively practiced a wire bonding method using a very fine wire of gold, aluminum or the like having a diameter of the order of several microns to several tens of microns. Particularly, a high purity gold wire having a purity of the order of 99.99% has been most commonly employed in such a bonding method because of its excellent corrosion-resistance, extensibility and wire bondability. The wire bonding method using such a gold wire has been usually conducted by a thermocompression bonding process. In the thermocompression bonding process, the gold wire is cut by an electrical means or by melting with a hydrogen flame to form a ball-shaped portion at the tip of the gold wire, the thus produced ball-shaped portion is connected onto an electrode of a semiconductor element by thermocompression and then the other end portion of the gold wire is connected to an external lead by thermocompression. These successive steps are all carried out at high temperature conditions of 200.degree. to 300.degree. C.
In recent years, attempts have been made for high speed and automatic bonding operation. However, when a conventional pure gold wire is used in the high-speed bonding process, it is subject to breaking or softening due to heat in the course of the operation, thereby being rendered unuseless. Therefore, there is a great demand for higher reliable bonding wires improved particularly in the following properties.
(1) They have a high mechanical strength, particularly a sufficiently high tensile strength at high temperatures, so that they are not subject to breaking during the bonding process. PA1 (2) A bonding wire loop connecting an electrode of a semiconductor element and an external lead does not droop by softening due to heat. PA1 (3) They are not subject to deformation by softening during the molding operation with resin. PA1 (4) The shape of a ball formed at the tip of the wire is almost completely spherical and uniform in size and thus scattering in the bonding strength is minimized.
Heretofore, many attempts have been made by adding various elements to the high purity gold wire in order to improve the properties as set forth above. For instance, Japanese patent Publication Nos. 57-34 659 and 58- 26 662 describe that strength can be improved by adding a very small amount of calcium or beryllium. Further, use of other many additives, such as platinum, palladium, silver, titanium, magnesium, etc., have been proposed.